Overcurrent protection is often implemented in class-D audio amplifiers, also known as switching amplifiers, to protect the system and connected components from damage that occurs from overcurrent conditions. FIGS. 1A and 1B illustrate an example embodiment of components comprising a typical class-D audio amplifier circuit 100, including switching amplifiers 102, low-pass filters 104, and output speaker 106. In such embodiments, conventional overcurrent protection is designed to detect an output current in excess of a threshold for which the circuit 100 shuts down to avoid damage to the device and other components such as the speaker 106. For example, when the output is shorted to ground 108, as illustrated in FIG. 1A, a large output current 110 is generated. The overcurrent protection feature detects this large output current 110 and shuts down the amplifier 100 if the output current 110 reaches the output current threshold set for the circuit 100.
When considering the dynamic behavior of a loudspeaker, however, it may be desirable to keep the circuit 100 operational to avoid an audible interruption in the output audio if the output current exceeds an output current threshold as a result of an impedance drop across the speaker 106, as opposed to a short-circuit condition. Thus, if the output current reaches or exceeds the output current threshold due to an impedance drop across the speaker 106 (see FIG. 1B), the output current 110′ is limited to a preset value to avoid shutting down the device 100, and the amplifier 102 continues switching to avoid an audible interruption in the output audio. This is known as current limiting.
It is often difficult to accurately distinguish between a short-circuit condition and an impedance drop using conventional overcurrent protection circuitry. Therefore, there exists a need for an overcurrent protection circuit capable of more accurately distinguishing between a short-circuit condition and an impedance drop and implementing appropriate protective measures in response thereto.